Index: clang/docs/ReleaseNotes.rst
===================================================================
--- clang/docs/ReleaseNotes.rst
+++ clang/docs/ReleaseNotes.rst
@@ -353,6 +353,9 @@
 - When the ``weak`` attribute is applied to a const qualified variable clang no longer
   tells the backend it is allowed to optimize based on initializer value.
 
+- New function attribute ``optsize`` can be used to perform optimizations designed to reduce
+  code size as long as they do not significantly impact runtime performance.
+
 Windows Support
 ---------------
 
Index: clang/include/clang/Basic/Attr.td
===================================================================
--- clang/include/clang/Basic/Attr.td
+++ clang/include/clang/Basic/Attr.td
@@ -2260,6 +2260,12 @@
   let Documentation = [OptnoneDocs];
 }
 
+def OptSize : InheritableAttr {
+  let Spellings = [Clang<"optsize">];
+  let Subjects = SubjectList<[Function, ObjCMethod]>;
+  let Documentation = [OptSizeDocs];
+}
+
 def Overloadable : Attr {
   let Spellings = [Clang<"overloadable">];
   let Subjects = SubjectList<[Function], ErrorDiag>;
Index: clang/include/clang/Basic/AttrDocs.td
===================================================================
--- clang/include/clang/Basic/AttrDocs.td
+++ clang/include/clang/Basic/AttrDocs.td
@@ -3456,6 +3456,16 @@
   }];
 }
 
+def OptSizeDocs : Documentation {
+  let Category = DocCatFunction;
+  let Content = [{
+This function attribute indicates that optimization passes and code generator
+passes make choices that keep the function code size low. The compiler also does
+optimizations to reduce code size as long as they do not significantly impact
+runtime performance.
+  }];
+}
+
 def LoopHintDocs : Documentation {
   let Category = DocCatStmt;
   let Heading = "#pragma clang loop";
Index: clang/include/clang/Sema/Sema.h
===================================================================
--- clang/include/clang/Sema/Sema.h
+++ clang/include/clang/Sema/Sema.h
@@ -3482,6 +3482,7 @@
                                           const AttributeCommonInfo &CI,
                                           const IdentifierInfo *Ident);
   MinSizeAttr *mergeMinSizeAttr(Decl *D, const AttributeCommonInfo &CI);
+  OptSizeAttr *mergeOptSizeAttr(Decl *D, const AttributeCommonInfo &CI);
   SwiftNameAttr *mergeSwiftNameAttr(Decl *D, const SwiftNameAttr &SNA,
                                     StringRef Name);
   OptimizeNoneAttr *mergeOptimizeNoneAttr(Decl *D,
Index: clang/lib/CodeGen/CodeGenModule.cpp
===================================================================
--- clang/lib/CodeGen/CodeGenModule.cpp
+++ clang/lib/CodeGen/CodeGenModule.cpp
@@ -1923,6 +1923,7 @@
       !CodeGenOpts.DisableO0ImplyOptNone && CodeGenOpts.OptimizationLevel == 0;
   // We can't add optnone in the following cases, it won't pass the verifier.
   ShouldAddOptNone &= !D->hasAttr<MinSizeAttr>();
+  ShouldAddOptNone &= !D->hasAttr<OptSizeAttr>();
   ShouldAddOptNone &= !D->hasAttr<AlwaysInlineAttr>();
 
   // Add optnone, but do so only if the function isn't always_inline.
@@ -1998,6 +1999,8 @@
       B.addAttribute(llvm::Attribute::Hot);
     if (D->hasAttr<MinSizeAttr>())
       B.addAttribute(llvm::Attribute::MinSize);
+    if (D->hasAttr<OptSizeAttr>())
+      B.addAttribute(llvm::Attribute::OptimizeForSize);
   }
 
   F->addFnAttrs(B);
Index: clang/lib/Sema/SemaDeclAttr.cpp
===================================================================
--- clang/lib/Sema/SemaDeclAttr.cpp
+++ clang/lib/Sema/SemaDeclAttr.cpp
@@ -4792,6 +4792,19 @@
   return ::new (Context) MinSizeAttr(Context, CI);
 }
 
+OptSizeAttr *Sema::mergeOptSizeAttr(Decl *D, const AttributeCommonInfo &CI) {
+  if (OptimizeNoneAttr *Optnone = D->getAttr<OptimizeNoneAttr>()) {
+    Diag(CI.getLoc(), diag::warn_attribute_ignored) << "'optsize'";
+    Diag(Optnone->getLocation(), diag::note_conflicting_attribute);
+    return nullptr;
+  }
+
+  if (D->hasAttr<OptSizeAttr>())
+    return nullptr;
+
+  return ::new (Context) OptSizeAttr(Context, CI);
+}
+
 SwiftNameAttr *Sema::mergeSwiftNameAttr(Decl *D, const SwiftNameAttr &SNA,
                                         StringRef Name) {
   if (const auto *PrevSNA = D->getAttr<SwiftNameAttr>()) {
@@ -4836,6 +4849,11 @@
     D->addAttr(MinSize);
 }
 
+static void handleOptSizeAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
+  if (OptSizeAttr *OptSize = S.mergeOptSizeAttr(D, AL))
+    D->addAttr(OptSize);
+}
+
 static void handleOptimizeNoneAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
   if (OptimizeNoneAttr *Optnone = S.mergeOptimizeNoneAttr(D, AL))
     D->addAttr(Optnone);
@@ -8533,6 +8551,9 @@
   case ParsedAttr::AT_OptimizeNone:
     handleOptimizeNoneAttr(S, D, AL);
     break;
+  case ParsedAttr::AT_OptSize:
+    handleOptSizeAttr(S, D, AL);
+    break;
   case ParsedAttr::AT_EnumExtensibility:
     handleEnumExtensibilityAttr(S, D, AL);
     break;
Index: clang/test/CodeGen/attr-optsize.cpp
===================================================================
--- /dev/null
+++ clang/test/CodeGen/attr-optsize.cpp
@@ -0,0 +1,79 @@
+// RUN: %clang_cc1 -Os -disable-llvm-passes -emit-llvm %s -o - | FileCheck %s -check-prefix=Os
+// RUN: %clang_cc1     -disable-llvm-passes -emit-llvm %s -o - | FileCheck %s -check-prefix=OTHER
+// RUN: %clang_cc1 -O1 -disable-llvm-passes -emit-llvm %s -o - | FileCheck %s -check-prefix=OTHER
+// RUN: %clang_cc1 -O2 -disable-llvm-passes -emit-llvm %s -o - | FileCheck %s -check-prefix=OTHER
+// RUN: %clang_cc1 -O3 -disable-llvm-passes -emit-llvm %s -o - | FileCheck %s -check-prefix=OTHER
+// Check that we set the optsize attribute on each function
+// when Os optimization level is set.
+
+__attribute__((optsize))
+int test1() {
+  return 42;
+// Os: @{{.*}}test1{{.*}}[[OPTSIZE:#[0-9]+]]
+// OTHER: @{{.*}}test1{{.*}}[[MS:#[0-9]+]]
+}
+
+int test2() {
+  return 42;
+// Os: @{{.*}}test2{{.*}}[[OPTSIZE]]
+// Os: ret
+// OTHER: @{{.*}}test2
+// OTHER-NOT: [[MS]]
+// OTHER: ret
+}
+
+int test3() {
+  return 42;
+// Os: @{{.*}}test3{{.*}}[[OPTSIZE]]
+// Os: ret
+// OTHER: @{{.*}}test3
+// OTHER-NOT: [[MS]]
+// OTHER: ret
+}
+
+// Check that the optsize attribute is well propagated through
+// template instantiation
+
+template<typename T>
+__attribute__((optsize))
+void test4(T arg) {
+  return;
+}
+
+template
+void test4<int>(int arg);
+// Os: define{{.*}}void @{{.*}}test4
+// Os: [[OPTSIZE]]
+// OTHER: define{{.*}}void @{{.*}}test4
+// OTHER: [[MS]]
+
+template
+void test4<float>(float arg);
+// Os: define{{.*}}void @{{.*}}test4
+// Os: [[OPTSIZE]]
+// OTHER: define{{.*}}void @{{.*}}test4
+// OTHER: [[MS]]
+
+template<typename T>
+void test5(T arg) {
+  return;
+}
+
+template
+void test5<int>(int arg);
+// Os: define{{.*}}void @{{.*}}test5
+// Os: [[OPTSIZE]]
+// OTHER: define{{.*}}void @{{.*}}test5
+// OTHER-NOT: define{{.*}}void @{{.*}}test5{{.*}}[[MS]]
+
+template
+void test5<float>(float arg);
+// Os: define{{.*}}void @{{.*}}test5
+// Os: [[OPTSIZE]]
+// OTHER: define{{.*}}void @{{.*}}test5
+// OTHER-NOT: define{{.*}}void @{{.*}}test5{{.*}}[[MS]]
+
+// Os: attributes [[OPTSIZE]] = { {{.*}} optsize {{.*}} }
+
+// OTHER: attributes [[MS]] = { {{.*}} optsize {{.*}} }
+
Index: clang/test/CodeGenObjC/attr-optsize.m
===================================================================
--- /dev/null
+++ clang/test/CodeGenObjC/attr-optsize.m
@@ -0,0 +1,12 @@
+// RUN: %clang_cc1 -emit-llvm -o - %s | FileCheck %s
+
+@interface Test
+- (void)test;
+@end
+
+@implementation Test
+- (void)test __attribute__((optsize)) {
+    // CHECK: define{{.*}}Test test
+    // CHECK: optsize
+}
+@end
Index: clang/test/Sema/attr-optsize.c
===================================================================
--- /dev/null
+++ clang/test/Sema/attr-optsize.c
@@ -0,0 +1,5 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+
+int foo(void) __attribute__((__optsize__));
+
+int var1 __attribute__((__optsize__)); // expected-warning{{'__optsize__' attribute only applies to functions and Objective-C methods}}